Structures in contact with bodies of water suffer from fouling and/or corrosion damage. For example the shipping industry has long faced serious problems caused by the adherence of marine organisms to ship hulls. Such fouling of a ship's hull increases the operating cost of a ship and decreases its efficiency.
Marine organisms which become attached to the hull must periodically be removed, thereby usually taking the ship out of operation for extended periods of time for dry dock maintenance. Also, if fouling is not prevented, aquatic organisms will continue to attach to the hull and will cause ever increasing operating costs associated with additional fuel requirements and decreased speeds. The pleasure boat market faces similar problems.
Several ways of removing marine organisms, including barnacle growth, from a ship are known. Barnacles can be mechanically scraped from the ship while in dry dock. Cleaning machines have been developed having rotating brushes which can remove barnacles and other marine organisms from the hull.
Another method of overcoming the fouling problems has been to use highly toxic paints on the hulls of ships. Such paints retard the buildup of marine growth on the hull. A toxic element in the paint, such as a compound of copper or mercury which is soluble in seawater, is controllably dissolved into the water to provide protection over several years. However, the leaching of toxic materials into esturine waters by a vast number of vessels, including the pleasure boat population, presents an increasing hazard to the environment.
For example U.S. Pat. No. 3,817,759 discloses the use of an antifouling coating comprising a polymeric titanium ester of an aliphatic alcohol. Titanium has good corrosion resistance and low water solubility which prevents premature leaching and exhaustion of the coating.
Another known antifouling method involves coating the hull of a ship with a metallic paint whose ions are toxic to marine life, i.e., copper, mercury, silver, tin, arsenic, and cadmium, and then to periodically apply a voltage to the hull to anodically dissolve the toxic ions into seawater thereby inhibiting marine life growth. This method is disclosed in U.S. Pat. No. 3,661,742 and in U.S. Pat. No. 3,497,434.
Antifouling systems which rely on dissolution of toxic substances into seawater have limited utility since the coating applied to the hull is depleted and the hull must be periodically repainted. The problem is made more severe in those systems which make the hull anodic to force dissolution since it increases the rate of dissolution. This poses a potentially serious problem since once the hull is exposed it too will be dissolved, resulting in pitting or puncturing of the hull.
Various other apparatus have been purposed which rely upon application of a voltage to the hull of the ship or provision for flow of current through the hull of the ship to retard growth of marine organisms on the hull. Some systems have proposed the electrochemical decomposition of seawater causing gases to be produced near the submerged surfaces of the hull.
Proponents of such systems maintain that the gases prevent the adherence of marine organisms such as barnacles, algae, etc. Others suggest that high current can cause shock and retard the growth of marine organisms on the hull. None of these systems, however, have proven commercially successful for reasons of cost and poor antifouling results. Examples of these systems are disclosed in U.S. Pat. No. 4,196,064 and Russian Patent No. 3388.
This problem is of course not limited to ships, but exists with all submerged structures capable of corroding.
Another aquatic animal, zebra mussels (Dreissena polymorpha), is posing major problems to electric utilities, and municipal and industrial facilities, that are dependent on raw waters, e.g., from the Great Lakes. The morphological, behavioral and physiological characteristics of zebra mussels promote rapid spread of the mussel within and between water bodies, colonization of natural and artificial structures, fouling of intakes, conduits, condensers, and piping systems, and resistant to on-line procedures typically used to maintain system reliability at fresh water power plants.
In the summer of 1989, the Electric Power Research Institute (EPRI) began to investigate the potential problems that can be caused by the zebra mussel and studied strategies for the utility industry to deal with these problems. The stimulus for this work was the rapid spread of the mussels, their impact on power plant operations, particularly those cited on Lake Erie, and concerns about current and future economic and ecological impacts.
Power plants offer prime habitats for zebra mussels. The plants contain a plethora of hard, relatively clean surfaces for mussels to colonize. This colonization is enhanced by the source and flow rate of water drawn into the plant. For example, most plants draw near-surface water where the larvae are found in the highest concentrations. In addition, flow rates specified at many intakes to prevent fish impingement are not high enough to prevent larval settlement. In fact, flowing water is advantageous for the settled mussels because it maintains food and dissolves oxygen concentrations necessary for sustenance. All power plant systems circulating raw water are vulnerable to zebra mussel fouling.
Large conduits, galleries and "boxes" can be subject to volume loss when mussels attach to the walls and each other forming mussel mats. These mats can reach thickness of several inches. Individual mussels can cause flow loss in small piping if flows are intermittent or slow enough for settlement or if mussels are transported to a construction. Even condensers are vulnerable to zebra mussel fouling. Only the very largest mussels have a shell height capable of blocking modern condenser tubing. However, mussel clusters, called druses, frequently break off from mussel mats. Such clusters have blocked up to 20% or more of the condenser tubes in a power plant on western Lake Erie.
To date, no satisfactory solution to this problem has been found. Large individual zebra mussels and mussel clusters can be removed by power plant traveling screens which serve to reduce their impact on cooling water systems. These screens are however not fine enough to remove early life stages (e.g., veliger larvae) which are capable of attachment in downstream locations inside power plants. The benefit of traveling screens is further reduced by large forebays that accommodate settlement and growth of mussel population. Physical filtration would require effective pore diameters on the order of 0.04 mm to retain the smallest larvae, and, as such, is impractical. By analogy to marine mussels, materials or coatings could theoretically be found that inhibit or prevent attachment of settling larvae. To date, none has yet been identified.
Another problem related to fouling of a ship's hull which the shipping industry has long attempted to solve is corrosion. Corrosion normally occurs to underwater portions of a ship's hull because the seawater acts as an electrolyte and current will consequently flow, as in a battery, between surface areas of differing electrical potential. The flow of current takes with it metal ions thereby gradually corroding anodic portions of the hull.
Various techniques have been developed to prevent corrosion. Sacrificial anodes of active metals such as zinc or magnesium have been fastened to the hull. Such anodes, through galvanic action, themselves corrode away instead of the hull.
Other systems use cathodic protection by impressed current. Such systems utilize long-life anodes which are attached to the hull to impress a current flow in the hull. The result is that the entire hull is made cathodic relative to the anode, thereby shielding it from corrosion. Such systems operate at very low-voltage levels, see, e.g., U.S. Pat. No. 3,497,434.
One known cathodic protection system utilizes a titanium anode plated with platinum. The platinum acts as the electrical discharge surface for the anode into the electrolytic seawater. No current is discharged from any surface portions of the electrode comprising titanium. This particular system impresses high current densities on the anode on the order of 550 amps per square foot. Since there is a high current flow from the platinum on other non-soluble anode metal, there is a very low potential and essentially no current flow from the surface of the titanium. An example of such a system is disclosed in U.S. Pat. No. 3,313,721.
A final problem faced by those desiring to develop a successful antifouling system is hydrogen embrittlement of the ship's hull. When electrolytic action takes place close to the surface of the ship's hull, such as in some of those systems described above, hydrolysis of the seawater may occur. Such hydrolysis releases hydrogen ions which cause embrittlement of the ship's hull. Consequently, it is important in any antifouling system which is installed that the system not be operated at such high current as to cause hydrolysis of the water thereby releasing hydrogen.
There is therefore a strongly felt need for a better method, and corresponding apparatus, for preventing the corrosion and/or fouling of structures which are fully or partially submerged in water.